Name: complex ions: metal and ligand stability
Uploaded: 2024-04-04T10:51:16.000Z
Duration: 1 min 9 s
Channel: JoVE

金属錯体の安定性は、中央の金属イオンと付着した配位子の性質に依存します。
そのため、金属に高い正電荷を持つ小さなイオンは、多くの配位子を強く引き付けて安定な錯体を形成します。
金属イオンは、クラスAとクラスBの金属に分類されます。
アルカリ、アルカリ土類、および最初の遷移系列の金属などのクラスA金属は、窒素、酸素、またはフッ素をドナー原子として他の配位子を持つ配位子と安定した錯体を形成します。
一方、クラスBは、2番目および3番目の遷移系列の金属と同様に、ドナー原子として硫黄または塩素を持つ配位子と安定した錯体を形成します。
かさばる配位子は、立体障害により金属-配位子結合を弱め、錯体を不安定にします。
あるいは、キレート配位子は金属中心と5員環または6員環を形成し、より安定した錯体を形成します。
さらに、3つ以上のドナー部位を持つ環状配位子は、大環状効果により非常に安定な錯体を形成します。

錯体形成平衡: 錯体の安定性に影響を与える要因

錯体形成反応では、金属カチオンが電子対受容体であり、配位子が電子対供与体です。金属錯体の安定性は主に中心金属イオンの錯形成能力と配位子の性質に依存します。一般に、金属イオンの錯形成能力は、イオンのサイズと電荷によって異なります。金属イオンとリガンドの価数が同じであれば、金属イオンのサイズが大きくなるにつれて、金属錯体の安定性は低下します。一方、金属イオンの電荷が増加すると、錯体の安定性が増加します。

金属イオンは、好ましい配位子、具体的には軟酸または硬酸および塩基の観点から記述される配位子に基づいてクラス A および B に分類されます。ソフト塩基は、分極率が高く、電気陰性度が低い、酸化しやすいドナー原子を持っていますが、ハード塩基は、分極率が低く、電気陰性度が高いドナー原子を持っています。不飽和の外側軌道を持つ、より小さく、より帯電した金属イオンはクラス A に分類され、硬い塩基に結合することを好みます。対照的に、外側軌道が満たされた、より大きく、電荷の少ない、または電荷のない金属イオンはクラス B に分類され、柔らかい塩基との錯体形成を好みます。錯体の安定性に対する配位子の影響に関しては、より小さく、より荷電の高い配位子はより安定な錯体を形成する傾向がありますが、より大きな配位子は立体反発の影響により安定性の低い錯体を形成する傾向があります。空の p 軌道または d 軌道を持つ配位子は、特定の金属イオンと pi 結合を形成することができ、その結果、配位子に対して逆結合挙動を示し、その両方が錯体をさらに安定化させる働きをします。

金属錯体の安定性は、中央の金属イオンと付着した配位子の性質に依存します。
だから、高い正電荷を持つ小さなイオン...

complexation-equilibria-factors-influencing-stability-of-complexes

Education

JoVE Core

Analytical Chemistry

分析化学

Unit 1

ユニット1

Chemical Equilibria

化学平衡

KEY TERMS AND CONCEPTS

主要用語と概念

Complexation Equilibria: Factors Influencing Stability of Complexes

complex ions: metal and ligand stability

Complex ions form when a metal cation accepts electron pairs from ligands, which are the electron pair donors. The stability of these metal complexes depends mainly on the metal ion and on the ligands attached to it.

The metal ion’s size and charge play a major role in stability. As the size of the metal ion increases, complex stability usually decreases when the valency of the metal ion and the ligands stays the same. As the charge on the metal ion increases, complex stability increases.

Metal ions are also grouped into Class A and Class B based on the ligands they prefer. This preference is described using the idea of hard and soft acids and bases. Soft bases have donor atoms that are easily oxidized, highly polarizable, and low in electronegativity. Hard bases have donor atoms with low polarizability and high electronegativity.

Smaller, more highly charged metal ions with unsaturated outer orbitals are Class A ions. They usually bind to hard bases. In contrast, larger metal ions with lower charge or no charge and filled outer orbitals are Class B ions, and they tend to complex with soft bases.

Ligands also affect stability through their size and bonding ability. Smaller ligands with a higher charge usually form more stable complexes. Larger ligands can form less stable complexes because steric repulsion, or crowding, makes binding less favorable.

Some ligands with vacant p or d orbitals can form pi bonds with certain metal ions. These ligands can also take part in back bonding. Both effects help stabilize the complex further.

Complex Ions: Metal and Ligand Stability

ionic strength: overview

Ionic Strength in Salt and Acid Solutions

Ionic strength describes the total electrolyte concentration in a solution. It is a quantitative measure of how many ions are present and how strongly they contribute to the solution. The concept was introduced in 1921 by Gilbert N. Lewis and Merle Randall while they were describing the activity coefficient of strong electrolytes. To calculate ionic strength, written as I or μ, both cations and anions are included. The concentration of each ion matters, but the charge number z matters even...

ionic strength: effects on chemical equilibria

ionic-strength-effects-on-chemical-equilibria

Salt Effect on Salt Solubility

Salt effect can change the solubility of a sparingly soluble salt in water. When an inert ionic compound is added, the dissolved salt may become more soluble. A common example is potassium nitrate added to a saturated calcium sulfate solution, which increases the amount of calcium sulfate that dissolves. Le Châtelier’s principle does not explain this shift well. Instead, the change is described by the effective concentration of ions in solution. Each ion is surrounded by an ionic atmosphere,...

thermodynamics: chemical potential and activity

thermodynamics-chemical-potential-and-activity

Chemical Potential in Solution Activity

Chemical potential and activity describe how a species behaves in a solution. Activity is the effective concentration of a species. It accounts for the electrolytes near that species and depends on the ionic strength of the solution. Activity is calculated as molar concentration multiplied by the activity coefficient. This makes it a more precise way to describe a species in solution than concentration alone. The thermodynamic equilibrium constant is also defined more accurately using activity...

thermodynamics: activity coefficient

Activity Coefficient in Ionic Solutions

Activity coefficient in ionic solutions describes how much a species behaves like its measured concentration. Activity is the effective concentration of a solute in solution. It equals the molar concentration multiplied by the activity coefficient, which is a unitless number. The activity coefficient depends on the total ionic strength of the solution. It shows how far a solution is from ideal behavior. When ionic strength is very low, the activity coefficient of an ionic species is close to...

chemical equilibria: redefining equilibrium constant

chemical-equilibria-redefining-equilibrium-constant

Salt Effect in Equilibrium Constant Calculations

Salt effect changes how a sparingly soluble salt dissolves when an inert salt is added. The size of this effect depends on the ionic strength of the solution. Ionic strength reflects how strongly the ions in the solution interact. In solution, the activity of a species is the product of its concentration and its activity coefficient. A redefined equilibrium constant must account for these activity effects when the ionic strength is moderately high. This form is also called the thermodynamic...

factors affecting activity coefficient

How Ionic Strength Changes Activity Coefficient

The activity coefficient of an ion in water depends on ionic strength, ion charge, and ion size. The extended Debye-Hückel equation describes these effects for aqueous solutions at 25°C. These three factors are partly linked, so changes in one can influence the others. When a solution has almost zero ionic strength, the activity coefficient is close to one. That means the solution behaves almost ideally. As ionic strength rises from 0 to 0.1 mol/L, the activity coefficient usually drops.

chemical equilibria: systematic approach to equilibrium calculations

chemical-equilibria-systematic-approach-to-equilibrium-calculations

Equilibrium Calculations with Mass and Charge

Chemical equilibrium calculations become easier when they are organized around mass balance and charge balance. This approach is useful for systems with multiple equilibria, where several reactions must be considered together. It is especially helpful when finding the solubility of a sparingly soluble salt in water with a common ion present. The first step is to identify every chemical reaction in the system. Next, write the equilibrium constant expression for each reaction. These expressions...

acid&#8211;base equilibria: activity-based definition of ph

acidbase-equilibria-activity-based-definition-of-ph

How Ionic Strength Changes pH

pH depends on hydrogen ion activity, not just hydrogen ion concentration. In an ideal solution, pH is the negative logarithm of the hydrogen ion concentration. In a non-ideal solution, a more accurate pH uses hydrogen ion activity, which combines concentration with an activity coefficient. This difference matters most in very low ionic strength solutions, such as pure water. At first, the hydrogen ion activity coefficient is close to one. When an electrolyte that does not donate or accept a...

ladder diagrams: acid&#8211;base equilibria

Using Ladder Diagrams to Predict pH

Ladder diagrams help show how acid-base equilibria change with pH. They are graphical tools that make the chemistry of a system easier to read during analysis. In a ladder diagram, the vertical axis shows pH. Horizontal bars, or steps, place the pKa values in the system.

ladder diagrams: redox equilibria

Redox Potential Shifts in Ladder Diagrams

Ladder diagrams show redox equilibrium using electrochemical potential, E, as the vertical scale. They help students see how changes in concentration affect which species is most stable. The areas of predominance are set with the Nernst equation. A useful example is the Fe3+/Fe2+ half-reaction, which has a standard-state potential of +0.771 V. At potentials more positive than +0.771 V, Fe3+ predominates. At potentials more negative than +0.771 V, Fe2+ predominates. Ladder diagrams also show...

ladder diagrams: complexation equilibria

Reading Metal-Ligand Ladder Diagrams

Ladder diagrams help show equilibria in metal-ligand complexes. In these diagrams, the vertical scale shows pL, or the concentration of free ligand that has not reacted. The horizontal lines mark the log of formation constants and show which species is dominant in each region. For the Cd and ammonia system, the formation constant K1 for Cd(NH3)2+ is 3.55 × 10^2. Its log value is 2.55, so pNH3 = 2.55 is the dividing line between Cd2+ and Cd(NH3)2+ dominance. Above 2.55, Cd2+ is the predominant...

solubility equilibria: overview

How Solubility Reaches Equilibrium

Solubility equilibrium describes what happens when a sparingly soluble salt dissolves in water and then reaches a balance with the solid left over. In a saturated solution, dissolved ions and the undissolved solid are in dynamic equilibrium. The process is reversible, so this topic is also called precipitation equilibrium. This balance matters in real life. Sugar-rich foods can produce organic acids that dissolve hydroxyapatite, the mineral in tooth enamel, which can lead to tooth decay. A...

solubility equilibria: ionic product of water

solubility-equilibria-ionic-product-of-water

Water Ionization and Kw in Equilibrium

Water ionization and the ionic product of water describe how pure water reaches equilibrium. Water is a weak electrolyte, so only a small amount breaks into hydrogen ions and hydroxide ions. At a fixed temperature, the amount of undissociated water stays almost constant. Because of this, the ionic product of water is the product of the hydrogen ion concentration and the hydroxide ion concentration. It is written as K w . The square root of K w gives the concentration of each ion in pure water.

complexation equilibria: overview

Stepwise Metal-Ligand Complexation

Stepwise metal-ligand complexation forms coordination compounds when metal ions and ligands share electrons through dative, or coordinate covalent, bonds. In these reactions, the metal ion binds to one or more ligands until its coordination number is reached. This number tells how many bonds form between the metal ion and the ligands. In aqueous solution, many metal ions are already surrounded by water molecules. These species are called aqua complexes. Complexation reactions are described by...

complexation equilibria: the chelate effect

complexation-equilibria-the-chelate-effect

Chelating Ligands and Metal Complex Stability

Metal complexes form when metal atoms or cations interact with ligands in a complexation reaction. These donor-acceptor adducts are central to understanding how metal ions bind in chemistry. Ligands can bind in different ways. A monodentate ligand uses one donor site. A bidentate ligand uses two donor sites, and a polydentate ligand uses more than two. Ethylene diamine is a bidentate ligand that binds through two nitrogen donor atoms and forms a five-membered ring. Polydentate ligands can...

complexation equilibria: factors influencing stability of complexes

Complex ions form when a metal cation accepts electron pairs from ligands, which are the electron pair donors. The stability of these metal complexes depends mainly on the metal ion and on the ligands attached to it. The metal ion’s size and charge play a major role in stability. As the size of the metal ion increases, complex stability usually decreases when the valency of the metal ion and the ligands stays the same. As the charge on the metal ion increases, complex stability increases. <br...

redox equilibria: overview

Redox Electron Transfer and Half-Reactions

Redox reactions involve the transfer of electrons from one species to another. In this type of reaction, electrons are not shared between atoms. They move from the species that loses them to the species that gains them. The species that loses electrons is the reducing agent, also called the reductant. It is oxidized during the reaction. The species that gains electrons is the oxidizing agent, or oxidant, and it is reduced. Redox reactions are often written as two half-reactions. One...

titrimetric methods: types and commonly used strategies

titrimetric-methods-types-and-commonly-used-strategies

Direct, Back, and Displacement Titrations

Titrimetric methods use a standard solution to measure the amount of an analyte in chemistry. The main categories are volumetric, gravimetric, and coulometric methods. Each one reaches the endpoint in a different way. In volumetric titrations, the key measurement is the volume of a titrant with a known concentration. The titrant is added until it reacts completely with the analyte. In gravimetric titrations, the standard solution forms an insoluble precipitate with the analyte. That solid is...

classification of titrimetric analysis based on reaction types

classification-of-titrimetric-analysis-based-on-reaction-types

Four Reaction Types in Titrimetric Analysis

Titrimetric analysis measures solution volume and is also called volumetric analysis. A standard solution with a known concentration is placed in the burette and is called the titrant. The solution with an unknown concentration in the flask is the analyte, or titrand. Titrimetric analysis is grouped into four types based on the reaction between the titrant and the analyte. Acid-base titrations involve a neutralization reaction between an acid and a base. This reaction forms water molecules.

key terms and concepts

Explore how metal ion size, charge, hard and soft bases, ligand size, and back bonding affect the stability of complex ions in chemistry.

Complex Ions: Metal and Ligand Stability | JoVE K12

JoVE Core 分析化学 は、複雑な化学原理をアニメーションで視覚化することで、SI 単位から NMR 分光法を含むトピックをよりわかりやすくします。実験を撮影した実写動画では、これらの概念が世界中の研究所でどのように応用されているかを取り上げています。

analytical chemistry

Complex Ions: Metal and Ligand Stability

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